Electrolytic apparatus for the production of alkali metal chlorate with grounding means

ABSTRACT

Electrolytic apparatus includes a power supply, at least two bipolar cells and at least one cell tank so arranged that the liquor in the cell tank is at ground potential, thereby avoiding the necessity and expense of insulating the cell tank from ground and also protecting certain components against corrosion.

nited States Patent Inventor Richard M. O. Maunsell Toronto, Ontario, Canada Appl. No. 875,257 Filed Nov. 10, 1969 Patented Nov. 30, 1971 Assignee Electric Reduction Company of Canada,

Ltd. llslington, Ontario, Canada Priority Nov. 8, 1968 Great Britain 52,999/68 ELECTROLYTIC APPARATUS FOR THE PRODUCTION OF ALKALI METAL CHLORATE WITH GROUNDING MEANS 5 Claims, 3 Drawing Figs.

US. Cl 204/196 lnt.Cl C23i' 13/00 Field of Search 204/204,

[56] References Cited UNITED STATES PATENTS 2,432,013 i2/l947 Hanson 204/228 X 2,435,973 2/i 948 MacTaggart et al 204/228 X FOREIGN PATENTS 741,31 1 8/1966 Canada 204/268 Primary Examiner-Winston A. Douglas Assistant Examiner-M. J. Andrews Attorney-Sim & McBurney ABSTRACT: Electrolytic apparatus includes a power supply. at least two bipolar cells and at least one cell tank so arranged that the liquor in the cell tank is at ground potential, thereby avoiding the necessity and expense of insulating the cell tank 7 from ground and also protecting certain components against corrosion.

ELECTROLYTIC APPARATUS FOR THE PRODUCTION OF ALKALI METAL CHLORATE WITH GROUNDING MEANS This invention relates to electrolytic apparatus for the production of an alkali metal chlorate and to power supplies therefor.

The production of sodium chlorate and the production of caustic/chlorine commonly is carried out by electrolysis of aqueous solutions of sodium chloride. In the production of caustic/chlorine, monopolar diaphragm or mercury cells are used almost universally, the total power supplied to each cell being relatively small, and the voltage applied across each cell being of the order of 4 or 5 volts. It is common practice to connect of the order of 150 such cells 'in series electrically, thereby permitting the use of an economical rectifier set having a DC voltage output of the order of 600 volts. With such an arrangement, the end cells of the line will be at potentials of +300 volts and 300 volts with respect to ground potential, and, in order to avoid current leakage between cells, it is common practice to mount the cells on porcelain insulators. This is not a difficult task because of the relatively small size and weight of such cells. 1

However, quite a different situation prevailsin the electrolysis of sodium chloride to produce sodium chlorate. The formation of chlorate requires that the anolyte and catholyte be mixed, so that diaphragm and mercury cathode of caustic chlorine cell are dispensed with, and a large number of bipolar electrodes that are electrically in series with each other are 1 employed. Because the conversion of hypochlorite, the primary product obtained when the anolyte and catholyte of a chlorate cell are mixed, to chlorate requires a large reaction volume, each chlorate cell and its associated cell tank are relatively large and may weight 200 tons or more. Such a cell generally requires about 120 volts and 1,500 kilowatts or more. It is common practice to connect six such cells in series electrically and supply the same from a single power producing 720 volts, the end cells of the line being 'at potentials of +360 volts and 360 volts with respect to ground.

The large cell tanks associated with the cells are difficult and expensive to insulate from ground either by means of porcelain insulators or by constructing the cell tanks themselves of insulating material.

In a line of six such cells there will be established what is referred to as a virtual ground. In a balanced system the virtual ground will be between the third and fourth cells in the line. However, if ground leakage increases in any cell, it is possible for the position of the virtual ground to shift. In an extreme case this could result in about 660 volts (liquor potential) to ground on one end cell and an intolerable increase in (l) the voltage stress applied to the cell and its components, (2) the current leakage and (3) excessive local heat evolution and destruction of the cell tank.

In accordance with one aspect of the present invention, the foregoing problems are eliminated by providing a system including a power supply, a cell and a cell tank wherein the potential of the liquor in the cell tank is ground potential.

This invention will become more apparent from the following detailed disclosure, taken in conjunction with the appended drawings, in which:

FIG. I is a schematic representation of the prior art system for electrolyzing an aqueous solution of sodium chloride to produce an aqueous solution of sodium chloride and sodium chlorate;

FIG. 2 is a schematic representation of one embodiment useful in explaining the instant invention; and

FIG. 3 is a schematic representation of a preferred embodi ment of the instant invention.

Referring to FIG. 1, there are shown six chlorate cells l0, l1, 12, I3, 14 and 15 and six cell tanks 20, 21, 22, 23, 24 and 25. Each cell is of conventional construction and includes a plurality of bipolar electrodes 16, liquor inlet tubes (not shown) and liquor outlet tubes (not shown). A DC power supply 17 has its positive terminal connected to one end electrode in cell 10 and its negative terminal connected to one end electrode in cell 15. If the cells are 120 volt cells, power supply 17 may be rated at, say, 760 volts, the voltage in excess of 720 volts being to compensate for wearing of the graphite electrodes as electrolysis proceeds. The end electrodes in the different cells are connected together as shown in FIG. 1 to thereby connect the cells in series electrically.

Cells 10-15 may be immersed in the liquor in cell tanks 20-25 respectively or may be outside of the cell tanks. In this event circulating pumps will be required to pump liquor from the cell tanks into the respective cells.

The sodium chloride solution may be introduced into cell tank 20 and flow through the other cells and cell tanks in series (cascade operation), or the cells may operate in parallel as far as liquor flow is concerned, or they may be batch operated.

In such a system as is shown in FIG. 1, cells 10 and 15 will be at potentials of +3 and 380 volts, and a virtual ground will be established between the third and fourth cells as shown at 18. The position of this ground can shift depending upon the operation of the various cells. Thus, for example, if cell 10 became grounded, the virtual ground would shift to cell 10, and the full voltage of the power supply would be applied between cell 15 and ground.

If the cells are within their respective cell tanks, it will be necessary to insulate the cell tanks from ground. If they are outside the cell tanks, both the cells and the cell tanks will have to be insulated from ground.

Referring now to FlG. 2, there is shown a 120 volt DC power supply, a bipolar chlorate cell containing bipolar electrodes 116 and a cell tank 200. The power supply is of a three-phase type and includes the secondary windings (Y connected) 101, 102 and 103 of a transformer and power rectifiers 104. The common terminal of the Y connected secondary windings is grounded, and the positive and negative terminals 105 and 106 respectively of the power supply are conventionally connected to monopolar electrodes 107 in cell 100. Cell tank 200 also is grounded.

With such an arrangement as is shown in FIG. 2, terminals 105 and 106 may be at +60 volts and -60 volts respectively. Since the liquor in cell 100 and cell tank 200 will assume a potential midway between +60 volts and 60 volts, i.e., 0 volts or ground potential, insulation of the cell and cell tank from ground is not necessary, and, indeed, the liquor in cell tank 200 can be grounded by grounding the cell tank, although this is not essential. The grounding of the common terminal of Y connected secondary windings 101-103 also is not essential, although it is preferred. It has the effect of minimizing small potential differences of the order of 1 volt which occur in cell tank 200 due to the submerged cell 100 and which aggravate corrosion of cooling coils in the cell tank liquor and other equipment.

The advantages of a system of the type shown in FIG. 2 are numerous. The problem of insulating the cell and/or the cell tank from ground is solved, because such insulation is not required. Malfunctioning of any one system will not change the voltage applied to the cell of any other system, it being understood that six systems of the type shown in FIG. 2 would replace the system of FIG. 1. A relatively inexpensive concrete-lined mild steel vessel can be employed for cell tank 200, since the vessel can be cathodically protected. Current leakage is no more of a problem than it is with any one of the cells of FIG. I when operating normally. Corrosion and deterioration of piping, pumps, cooling coils, cell tanks and cell boxes is materially reduced.

A disadvantage of the system of FIG. 2 is the cost of six lowvoltage power supplies (6 X volts X 12,000 amps) as compared with the cost of one 760 volt X 12,000 amps power supply. However, three 760 volts X 4,000 amps power supplies have been found to cost no more than a conventional single power supply having the same total capacity, particularly as there is a considerable saving in copper bus bars, as is the case with the system of FIG. 3. However, 760 volts cannot be applied across a single bipolar chlorate cell without creating intolerable current leakage and voltage stress problems. Any attempt to solve the current leakage problem necessarily will reduce the rate of natural gas lift liquor circulation in the cell and hence reduce current efficiency and increase cell voltage, thus providing a lower output at a higher power cost. Some idea of the magnitude of the current leakage problem can be appreciated from the fact that current leakage normally accounts for a 2 percent inefficiency, and, other things being equal, is proportional to the square of the voltage. Thus, merely increasing the number of unit cells in a submerged cell so that it can absorb 760 volts rather than 120 volts without other redesign will increase current inefficiency from 2 percent to 2 (760l 20) =80 percent.

In the embodiment of this invention shown in FIG. 3, the aforementioned current leakage problem is solved and yet an economical power supply (760 volts X 4,000 amps, for example) can be employed.

Referring to FIG. 3, there is provided a power supply like the power supply of FIG. 2 but rated at, say, 760 volts and 4,000 amps rather than 120 volts and 4,000 amps.

The cell itself is divided into two sections 100a and 1001), and each section is insulated from ground by porcelain or other insulators 108. This creates no great problem, since each cell section would weight of the order of tons rather than of the order of 200-300 tons, the weight of both a cell and a cell tank. Each cell section contains monopolar and bipolar (not shown) electrodes 107 and 116 respectively arranged as shown in FIG. 2, and the cell sections are connected in series electrically by bus bar 109. Across each half cell section is a potential of 380 volts, the electrode 107 of half cell section 100a to which the power supply is connected being at +380 volts and the electrode 107 of half cell section 1001) to which the power supply is connected being at 380 volts.

Associated with each half cell section is an inlet manifold 130 to which cell liquor is supplied from common cell tank 200 for distribution to the various unit cells. Also associated with each cell section is an outlet manifold 131 to which liquor from the various unit cells is supplied and which delivers this liquor to cell tank 200. Cell tank liquor is supplied to manifolds 130 by a pump 132 and pipes 133 and 134, the liquor first passing through a cooler 135 or heat exchanger. The liquor is returned to cell tank 200 from manifolds 131 via pipes 136 and 137 and a gas separator 138 from which hydrogen is withdrawn via a pipe 139. It will be seen that the liquor flow paths to and from the two cell sections are in parallel with each other.

it should be noted that the components designated 132, 135 and 138 can be duplicated, if desired, and a separate cell tank for each half cell section employed in the interest of more cient use of the graphite or other electrodes in cascade operation when a small number of the cells are required.

Bus bars 109 and pipes 136 and 134 are grounded, thereby ensuring that the cell tank liquor will be at ground potential =and thus protecting pump 132, cooler 135, the pipes. gas separator 138 and cell tank 200.

While one end of each half cell section is at a potential 380 volts different from ground potential, the half cell sections are insulated by insulators 108. Provided that insulators 108 are kept clean, the maximum potential stress over the surface of or through the material of each half cell section is 3 volts per inch as compared with 240 volts per inch for presently used cells and 2,500 volts per inch for presently used cell tanks. Obviously, the potential stress in cell tank 200 is zero.

lt should be noted also that in the arrangement of FIG. 3 where the pump, cooler and cells are not present in cell tank 200, there is no need to ground the common terminal of Y connected secondary windings 101-103.

It also should be noted that other cells could be electrically connected in series with cells a and 100b, but it would be necessary for either group of cells to be supplied directly with liquor from and to return directly liquor to a common cell tank for that group of cells, which common cell tank also may be the common cell tank for the other group of cells.

What I claim as my invention is:

l. Electrolytic apparatus for electrolyzing an alkali metal chloride to produce an alkali metal chlorate comprising: a DC power supply having positive and negative terminals relative to ground potential; first and second electrolytic cells each including a housing and, within said housing, first and second monopolar electrodes and a plurality of bipolar electrodes located between said monopolar electrodes; means electrically connecting said positive terminal to said first monopolar electrode of said first cell; means electrically connecting said negative terminal to said second monopolar electrode of said second cell; means electrically grounding said second monopolar electrode of said first cell and said first monopolar electrode of said second cell to maintain said second monopolar electrode of said first cell and said first monopolar electrode of said second cell at ground potential; means insulating said cells from ground; at least one cell tank adapted to contain liquor to be electrolyzed; at least one inlet manifold for supplying liquor from said cell tank to said cells; at least one outlet manifold for receiving liquor from said cells; means for delivering liquor from said cell tank to said inlet manifold; means for delivering liquor from said outlet manifold to said cell tank; and means grounding liquor in said cell tank.

2. The invention according to claim 1 wherein said inlet and outlet manifolds are grounded.

3. The invention according to claim 2 wherein there is only one of said cell tanks.

4. The invention according to claim 2 wherein said means for delivering liquor to said inlet manifold include a pump and a cooler and said means for delivering liquor from said outlet manifold include a gas separator.

5. The invention according to claim 3 wherein there are only two of said cells. 

2. The invention according to claim 1 wherein said inlet and outlet manifolds are grounded.
 3. The invention according to claim 2 wherein there is only one of said cell tanks.
 4. The invention according to claim 2 wherein said means for delivering liquor to said inlet manifold include a pump and a cooler and said means for delivering liquor from said outlet manifold include a gas separator.
 5. The invention according to claim 3 wherein there are only two of said cells. 